I think 18 is respectable for all legal purposes. Most kids are drinking at that age anyway.
Age limits exist currently for the same reasons speed limits and such do: we provide the limits because otherwise you lack the skill &/or discipline to avoid killing yourself. So, certainly, with the lowered age should come social measures to change behaviours of those willing to drink. As well, expect deaths-and those will fuel the social change.
I am certainly unafraid to drink, and to excess sometimes. If I know I will be drinking, I don't drive to the club or bar or restaurant(ha, or speakeasy). I take a taxi. That way I cannot make a bad, drunken decision to get behind the wheel.
Designated drivers are another option that are not, unfortunately, utilized enough.